Lasers can be used to deliver light to biological or non-biological particles and emission spectra can be used for the analysis of particle characteristics. In some instances, this can be applied such as where a particle is self florescent or self color absorbing, is associated by affinity, avidity, covalent bonds, or otherwise to another molecule which may be colored or fluorescent, may be associated to another molecule which is colored or fluorescent through a specific biological or modeled macromolecular interaction, such as an antibody binding event or a nucleic acid oligomer or polynucleic acid hybridization event, may obtain color or fluorescence such as through an enzymatic synthesis event, an enzymatic attachment or cleavage reaction, enzymatic conversion of a substrate, association of a florescent molecule with a nearby quencher, the reaction of a product in certain local proton (pH) or NADH or NADPH or ATP or free hydride (H—) or bound hydride R—(H—) concentrations, or may gain color or fluorescense by way of a variety of methods to associate emitted or absorbed light (electromagnetic radiation EMR).
Conventional lasers can generate a strong, perhaps intense, source of light. Through coherence properties of the beam such light may travel very long distances, perhaps across reflective mirrors which may change the angle of the light illumination beam, perhaps through prisms or refractive objects or lenses which may split it into two or more beams of equal or differing intensity, or may defocus, perhaps expand, or focus, perhaps concentrate, the beam. Such light may also be affected by filters which may reduce the net energy of the beam. Most lasers also allow the modulation of light intensity, perhaps watts, in the beam by adjustment of an input current from a power supply to the light generating element.
In some applications, conventional lasers used in the analysis and quantification of biological objects can be combined with sensitive light detectors that may be as simple, such as a photographic film or paper, or may be more complex, such as a photomultiplier tube. Often, a light detector may collect only information about a cumulative amount of light, perhaps electromagnetic radiation, EMR, or it may collect and report on the dynamic changes in intensity of light or EMR hitting all of, or portions of, localized regions of, or positions on the detector surface. The light detector may also involve use of a photoelectric coupling device, which may allow the energy of photons absorbed on the EMR by the light detector to be converted to current proportional to the incident light or EMR on the light detector surface. The photoelectric coupling device can even be integrated into an electronic circuit with an amplifier which may increase the signal or create gain such that the fluctuations or perhaps summation of amplified current may be available to an analog or digital logic circuit. Designs may also transmit a signal or data set to a user of a particle analysis instrument and this signal may be proportional to the static, cumulative, or perhaps even dynamic intensity of the light or EMR incident upon the detector.
In certain uses of laser light to analyze biological particles, a detector may measure the change in intensity of the source light after incidence upon a particle(s) being analyzed using a reference beam which takes a path without incidence upon the particle(s). In other uses of laser light, modified or unmodified particles take up a fraction of the illumination light or EMR and may emit light of a different frequency. In many cases, the presence of emission light or EMR of a certain wavelength can be used to identify or to quantify characteristics associated with specific particles, or quantitatively measure the amount or number of the specific biological particles present in the sample or in a specific region of or position in the sample.
In some cases, it can be useful to accurately determine very small differences in the illumination light or emission light from two very similar biological particles (for example an X-chromosome bearing sperm cell versus a Y-chromosome bearing sperm cell). These small differences can be analyzed by way of serial presentation of perhaps 50,000 separate emission events per second in a liquid stream. These can also be thousands of separate emissions from molecules (nucleic acids or proteins as examples) on an array field allowing analysis of genetic, genomic, proteomic, or glycomic libraries.
The traditional type of laser used for the analysis of particles in flow cytometry is a continuous wave (CW) laser. Often this provides a beam of constant intensity. However, in some instances, CW lasers can have particular disadvantages for applications as discussed here. The beam can result in modification or destruction of the sample being observed. For example, with respect to sperm cells, irradiation can result in lower fertility of the sperm cells. Second, in some instances when the laser beam continuously operates, it may be desirable to have a method of interrupting the beam if it is moved from a first location of incidence to a second location of incidence without illumination of intermediate areas.
In U.S. Pat. No. 5,596,401 to Kusuzawa, a pulsed laser may be used for imaging an object, such as a cell, in a flow cytometer. This disclosure may be related to improvements in the capture of images from particles such as coherence lowering modulations. Kusuzawa may teach a use of a continuous wave laser for particle detection and imaging.
In U.S. Pat. No. 5,895,922 and U.S. Patent Application No. 2003/0098421 to Ho, pulsed laser light may be used to illuminate and detect hazardous biological particles dispersed in an airflow stream. The invention may include an ultraviolet laser light and looking for the emission of fluorescence from potentially hazardous biological particles. This disclosure may teach the disadvantages of a laser diode apparatus.
U.S. Pat. No. 6,177,277 to Soini, describes employing a two-photon excitation and/or confocal optimal set-up. The invention may relate to the use of confocal optics to reduce an analysis volume to about 10% of standard analysis volume in a flow cytometer. A pulsed laser may provide short pulses of intense light and may allow the simultaneous absorption of two photons so that a wavelength of illuminating light beam may be longer than an emitted single photon bursts. Background signal may be reduced by use of a filter. The invention may include dual signal processing. The invention as described in Soini, may be beneficial in the analysis of small particles such as erythrocytes and bacterial cells.
In U.S. Pat. No. 6,671,044 to Ortyn, a special analysis optics and equipment may be used in an imaging flow cytometer. The Ortyn disclosure may include analyzing a sex of fetal cells in maternal blood as a method for determining the sex of a child during early pregnancy. Ortyn may indicate that analysis rates from an imaging flow cytometer may be restricted to theoretically maximizing at 500 cells per second.
With respect to particle analysis using laser light, the present invention discloses technology which addresses each of the above-mentioned problems.
For the purposes of this invention, a rapidly pulsed, high intensity pulsed laser may be used. This laser may deliver short pulses of high intensity perhaps lasting about 5-20 picoseconds, followed by intervals between pulses which are 100-1000 times as long as the pulses or about 0.5-20 nanoseconds. The light may have very high peak intensities over the period of about 5-20 picoseconds, and low net energies over the period of about 2-10 microseconds.
Flow cytometry, using a high-speed cell analysis, or high-speed cell analysis and sorting instrument, often relies on a laser light source to illuminate a stream of fluid in which particles are entrained. Particles may be caused to flow by a point of illumination at a rapid rate, often in the range of 500 to 100,000 particles per second. Often the light from the illuminating laser source is of constant intensity. The particles in the analysis stream may be of the same size, and may spend the same amount of time within the area of illumination. The amount of light illuminating each particle in a large population of particles analyzed in series may be identical. A detector may be capable of measuring scattered light, or other types of light emitted by the particle as a result of auto-fluorescence or fluorescence associated with a chemical dye, dye complex, or conjugated dye which may be targeted to one or more types of molecular species contained on or within particles in the population and can determine the identity of a particle and, in some cases, make a measurement of the quantity of a specific molecular target associated with the particle. A specific molecular structure on or even within a particle may be characterized and a quantitative measurement of the amount of associated molecular structure on or even within a particle, may yield information which may be used as a basis for sorting out or separating one type of particle from another.
In a flow cytometer, there may be a very short time duration between the exact moment that a particle is illuminated and the exact moment that a physical manipulation or an electrical condition, may be triggered to elicit separation of a specific particle from a stream containing various particles. An example of a physical manipulation may be charging of a droplet. A specific duration may be called a drop delay period, and the duration may be perhaps as brief as about 100 microseconds or perhaps as long as about 10 milliseconds, and may even be about 1 millisecond. In the case of particle sorting, information may be detected from each particle, computational analysis of the information may be determined, and comparison of the computation to a gating value or perhaps even a selection criteria may be accurately performed within a time period shorter than a duration of the drop delay.
Flow cytometer systems may be useful for measuring an average amount of a specific molecule present on or even within a population of particles. Past systems may not have measured the exact amount of a specific molecule on or even within a population of particles. Factors which can contribute to inaccurate measurements of single particles may include the saturation of a stain or even a conjugate to a particle, variation in the quanta of illumination light, effects from the shape of a particle, and perhaps even electronic noise in the detection apparatus.
An example of a particularly challenging problem is the sorting of X-chromosome bearing and Y-chromosome bearing sperm of mammals at high processing rates and high sorting purities. The population of sperm in most mammals is about 50% X-chromosome bearing and about 50% Y-chromosome bearing. A stain, such as Hoechst 33342, may form complexes with double stranded DNA. A measurement of total Hoechst 33342-DNA complex in each sperm may correlate to the total amount of DNA in each sperm. In general, mammals have larger X chromosomes than Y chromosomes and may have a differential between total DNA contents of X-chromosome bearing over Y-chromosome bearing sperm for various mammals. Such differentials may include: human having about 2.8%; rabbit having about 3.0%; pig having about 3.6%; horse having about 3.7%; cow having about 3.8%; dog having about 3.9%; dolphin having about 4.0%; and sheep having about 4.2%. The differentials may correlate to a relative difference of intensities emitted from a stained sperm being sorted for the purpose of separation of X-chromosome bearing and Y-chromosome bearing sperm.
Significant achievements have been made in developing staining conditions to stain DNA in live sperm with Hoechst 33342, such as, the use of dual orthogonal detection systems to determine sperm orientation, the use of hydrodynamic fluidics to increase the numbers of correctly oriented sperm, the setting of gain on detectors, and even the use of high-speed electronics. In the most efficient use of said achievements, it may be possible in most mammals to simultaneously sort sperm into two populations, X-chromosome bearing, and Y-chromosome bearing, at rates of 2500 per second or higher. It may also be possible to sort sperm to purities of 90% or even higher. There may be, however, a distinct problem in that at rates faster than 2500 per second, the purity of the sample may decrease.
This problem may be understood due to the observation that the co-efficient of variation (CV) in possibly even the best sperm sorting procedures may be between about 0.7%-1.5%, and with poor conditions can even be between about 2%-5%. Since the difference in DNA between X-chromosome bearing sperm and Y-chromosome bearing sperm in mammals may be as low as 2.8% as seen in humans and as high as 7.5% as can be seen in chinchillas, the CV may be lower than the DNA differential in order to achieve a large enough separation of the two populations. Humans have one of the lowest known DNA differentials and may have some of the lowest known maximum purities in sorting. It may be desirable to improve procedures which can reduce the CV.
A method which has been shown to improve the CV may be to use higher intensities of laser light illumination. For example, it is known to use continuous wave lasers to sort various sperm species with between about 100-200 milliwatts of laser illumination, and possibly with about 150 milliwatts. It has been observed that doubling or tripling the intensity and increasing the power to about 300-500 mW can improve the CV. An improved CV can be most apparent by analysis of the “split” between two peaks on a histogram. Yet, there may be problems associated with an increase of intensity or perhaps even an increase of power with a continuous wave laser. In the case of analyzing a Hoechst 33342 DNA complex with a continuous wave laser, the light source may be near a UV spectrum and may have some ionizing effect upon the DNA complex. Ionizing may then cause changes to the DNA. Accordingly, sperm sorted with high intensities continuous wave lasers such as 300-500 mW may not be as fertile. Another problem may include the energy that it may take to power a continuous laser to deliver about 150 mW of energy at near UV spectrum. Continuous wave lasers may require 10,000 mW or perhaps even more of power. Since there may already be a large amount of electrical power required to run a continuous wave laser at 150 mW, a much larger amount of power may be required to run a continuous wave laser at higher powers. Furthermore, a tube life of ion lasers may be reduced when operating at higher powers. An additional problem with the use of continuous wave lasers may be that the CV may drop significantly when using lower powers such as between about 20-80 mW.
In embodiments, the present invention provides flow cytometer designs which may incorporate the use of 2 or more flow nozzles, and even as many as dozens of flow nozzles, possibly operated by a single sorting instrument. Fields such as microfluidics, optics, electronics, and even parallel processing may be explored. In other embodiments the present invention includes the use of beam splitters to create multiple light beams. Yet, a major problem facing the development of reliable flow analysis and flow sorting in parallel may be the high intensity of laser light needed for analysis at each nozzle. This problem is particularly relevant for applications which require a very low CV in measurement of identical particles.
There is a need to provide flow systems for the analysis and sorting of particles that require a low CV value, yet may require higher laser light intensities, yet higher intensities may have negative effects on sperm and require higher power. In the search for solutions to the problems in flow systems for the analysis and sorting of particles, the field of pulsed lasers represents a possible solution.
Surprisingly, even though sperm sorted on a high speed flow cytometer may be damaged by UV light between about 300-500 mW, it is now shown in this invention that powers between about 100-500 mW may not be damaging to sperm if they are delivered in pulses. In embodiments, this may include a pulse having a peak intensity possibly as much as 1000 times higher than the intensity of a continuous wave laser. Pulsed lasers may be designed as quasi-continuous wave lasers and may have fast repetition rates such as between about 50-200 Megahertz and even up to 80 Megahertz. In embodiments, pulses may be between about 5-20 picoseconds. Pulsed lasers may be ideal for providing pulsed light to a stream of particles being analyzed in a flow cell or a flow cytometer. Particles analyzed in flow cytometers with event rates possibly between 10,000-100,000 Hertz, and even between 20,000-60,000 Hertz, may be illuminated from a few hundred pulses from a laser having repetition rates near 80,000 Hertz. Each pulse may provide an intense amount of energy.
There may be certain industrial uses of flow cytometers, as preparative instruments, which may be economically limited by the traditional methods of processing. It may be desirable to provide systems which facilitate parallel processing for industries such as those that rapidly process mammalian ejaculates for the production of large numbers of live sperm for insemination, those that process blood samples for the recovery of specified cells such as fetal cells, white blood cells, stem cells, hematopoetic cells from bone marrow, and the like. In an embodiment of the present invention, special forms of pulsed laser light can allow a single laser to illuminate a plurality of nozzles, perhaps even while not reducing the CV of the samples analyzed.
As a result, by the use of special forms of pulsed laser light, further improvements in the speed and sample purity can be seen. These types of lasers may be essential in the design and development of new flow cytometers perhaps having multiple sorting streams as well.